Process for large-scale production of cdte/cds thin film solar cells

ABSTRACT

A process for large-scale production of CdTe/CdS thin film solar cells, films of the cells being deposited, in sequences on a transparent substrate, the sequence comprising the steps of: depositing a film of a transparent conductive oxide (TCO) on the substrate; depositing a film of CdS on the TCO film; depositing a film of CdTe on the CdS film; treating the CdTe film with CdCl 2 ; depositing a back-contact film on the treated CdTe film. Treatment of the CdTe film with CdCl 2  comprises the steps of: forming a layer of CdCl 2  on the CdTe film by evaporation, while maintaining the substrate at room temperature; annealing the CdCl 2  layer in a vacuum chamber at a temperature generally within a range of 380° C. and 420° C. and a pressure generally within a range of 300 mbar and 1000 mbar in an inert gas atmosphere; removing the inert gas from the chamber so as to produce a vacuum condition, while the substrate is kept at a temperature generally within a range of 350° C. and 420° C. whereby any residual CdCl 2  is evaporated from the CdTe film surface.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of the solar cells technology and more particularly concerns a process for the large-scale production of CdTe/CdS thin film solar cells.

BACKGROUND ART OF THE INVENTION

[0002] As is known, a typical configuration of a CdTe/CdS solar cell has a film sequence of the multi-layer arrangement comprising a transparent glass substrate carrying a transparent conductive oxide (TCO) film, a CdS film representing the n-conductor, a CdTe film representing the p-conductor and a metallic back-contact. A solar cell with a layer arrangement and structure of this type is disclosed, for example, in U.S. Pat. No. 5,304,499.

[0003] The commercial float glass may be used as a transparent substrate, but, in spite of its low cost, special glasses are often preferred to avoid drawbacks of the float glass, in particular Na diffusion into TCO film.

[0004] The most common TCO is In₂O₃ containing 10% of Sn (ITO). This material has a very low resistivity on the order of 3×10⁻⁴ Ωcm and high transparency (>85%) in the visible spectrum. However, this material is made by sputtering and the ITO target after several runs forms some noodles which contain an In excess and a discharge between noodles can happen during sputtering which can damage the film. Another material which is commonly used is fluorine doped SnO₂ which however exhibits a higher resistivity close to 10⁻³ Ωcm and as a consequence a 1 μm thick layer is needed in order for the sheet resistance to be around 10 Ω/_(square). A high TCO thickness decreases the transparency and then the photocurrent of the solar cell. Finally a novel materia+03−1, namely Cd₂SnO₄ has been developed by the NREL group (X. Wu et al., Thin Solid Films, 286 (1996) 274-276)). Also this material has some drawbacks since the target is made up of a mixture of CdO and SnO₂ and, being CdO highly hygroscopic, the stability of the target may result to be unsatisfactory.

[0005] The CdS film is deposited by sputtering or Close-Spaced Sublimation (CSS) from CdS granulate material. This last technique allows the preparation of thin films at a substrate temperature much higher than that used in simple vacuum evaporation or sputtering, because substrate and evaporation source are put very close to each other at a distance of 2-6 mm and the deposition is done in the presence of an inert gas such as Ar, He or N₂ at a pressure of 10⁻¹-100 mbar. A higher substrate temperature allows the growth of a better crystalline quality material. An important characteristic of the close-spaced sublimation is a very high growth rate up to 10 μm/min, which is suitable for large-scale production.

[0006] CdTe film is deposited on top of CdS film by close-spaced sublimation (CSS) at a substrate temperature of 480-520° C. CdTe granulate is generally used as a source of CdTe which is vaporised from an open crucible.

[0007] An important step in the preparation of high efficiency CdTe/CdS solar cells is the treatment of CdTe film with CdCl₂. Most research groups use to carry out this step by depositing on top of CdTe a layer of CdCl₂ by simple evaporation or by dipping CdTe in a methanol solution containing CdCl₂ and then anneal the material in air at 400° C. for 15-20 min. It is generally believed that the CdCl₂ treatment improves the crystalline quality of CdTe by increasing the size of small grains and by removing several defects in the material.

[0008] After CdCl₂ treatment, CdTe is etched in a solution of Br-methanol or in a mixture of nitric and phosphoric acid. Etching is necessary as CdO or CdTeO₃ are generally formed on the CdTe surface. CdO and/or CdTeO₃ have to be removed in order to make a good back contact onto CdTe. Besides it is believed that, since etching produces a Te-rich surface, the formation of an ohmic contact when a metal is deposited on top of CdTe is facilitated.

[0009] The electric back contact on the CdTe film is generally obtained by deposition of a film of a highly p-dopant metal for CdTe such as copper, e.g. in graphite contacts, which, upon annealing, can diffuse in the CdTe film. The use of a Sb₂Te₃ film as a back-contact in a CdTe/CdS solar cell has been disclosed by the same applicants (N. Romeo et al., Solar Energy Materials & Solar Cells, 58 (1999), 209-218)

[0010] Industrial interest towards thin films solar cells is increased in recent years also in view of the high conversion efficiency reached so far. A record 16,5% conversion efficiency has been recently reported (see X.Wu et al., 17^(th) European Photovoltaic Solar Energy Conversion Conference, Munich, Germany, 22-26 Oct. 2001, II, 995-1000). Therefore several efforts have been made to provide processes suitable for large-scale, in-line production of CdTe/CdS thin film solar cells.

[0011] A state-of-the-art report concerning this issue may be found in D. Bonnet, Thin Solid Films 361-362 (2000), 547-552. However, a number of problems still hinder the achievement of this result, in particular concerning some crucial steps which affect either stability and efficiency of CdTe/CdS thin film solar cells or their costs.

[0012] A major problem of the known processes is the etching step to which the CdTe surface must be submitted to remove CdO or CdTeO₃ oxides formed thereon. Since etching requires the immersion of substrates carrying the treated CdTe/CdS films into acid solutions, rinsing and drying, machinery suitable for a continuous operation presently does not exist. Another significant problem which negatively affects the stability of the TCO films, as well as the cost of the final product are the drawbacks presently encountered with the use of known TCOs, as previously mentioned. In addition to these drawbacks, known TCOs require the use of special glasses, such as borosilicate glass, to avoid the problem of Na diffusion, occurring if a soda-lime glass is used, which would damage the film.

[0013] A further problem concerns the source from which the CdS film and the CdTe film are produced by close-spaced sublimation. When small pieces of these materials containing dust are used as a sublimation source, due to a different thermal contact, some micro-particles can be overheated and then split on to the substrate together with the vapour. In order to avoid this inconvenience, complicated metallic masks are used in some cases, which make a continuous operation problematic.

OBJECT AND SUMMARY OF THE INVENTION

[0014] It is the main object of the present invention to provide a process suitable for a large-scale production of stable and efficient CdTe/CdS thin film solar cells on a low cost substrate.

[0015] A particular object of the present invention is to provide a process of the above mentioned type in which the treatment of the CdTe film with CdCl₂ is conducted in such a way as not to require an etching treatment to remove the oxides possibly formed on the CdTe film.

[0016] A further object of the present invention is to provide a process of the above mentioned type, in which the deposition of the TCO film is conducted in such a way that a film of very low resistivity can be deposited without formation of any metal noodles on the target and allowing the use of a inexpensive substrate.

[0017] Still another object of the invention is to provide a process of the above-mentioned type, which allows the formation of CdS and CdTe films completely free of dusts.

[0018] A further object of the present invention is to provide a stable, efficient and relatively low-cost CdTe/CdS thin film solar cell.

[0019] The above object are achieved with the process for the large scale production of CdTe/CdS thin film solar cells, the main features of which are set forth in claim 1.

[0020] According to an important aspect of the invention, the treatment of the CdTe film with CdCl₂ is carried out by first forming a 100-200 nm thick layer of CdCl₂ on the CdTe film by evaporation, while keeping the substrate at room temperature; then annealing the CdCl₂ layer in a vacuum chamber at 380-420° C. and 300-1000 mbar under inert gas atmosphere; and finally removing the inert gas from said chamber to produce vacuum condition, while the substrate is kept to a temperature of 350-420° C., whereby any residual CdCl₂ is evaporated from the CdTe film surface. In this way, there is no need for the etching treatment of the CdTe film and the process can be conducted in a continuous way.

[0021] According to another aspect of the invention, the TCO layer is formed by sputtering in an inert gas atmosphere containing 1-3 vol. % hydrogen and a gaseous fluoroalkyle compound, in particular CHF₃. In this way the TCO is doped whit fluorine

[0022] According to another important aspect of the invention, as a source material for the formation of the CdS and the CdTe films by sputtering or close-spaced sublimation, a Cds or, respectively, CdTe material in the form of a compact block is used.

[0023] Further features of the process according to the invention are set forth in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Further features and advantages of the process for large-scale production of CdTe/CdS thin film solar cells according to the present invention will be apparent from the following description of a preferred embodiment made with reference to the attached drawings, wherein:

[0025]FIG. 1 is a schematic representation of the film sequence of the CdTe/CdS thin film solar cells according to the invention;

[0026]FIG. 2 is a schematic diagram of the process according to the invention.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0027] With reference to the figures+, the CdTe/CdS solar cells produced with the process according to the invention comprise five layers deposited in a sequence on a transparent base layer or substrate and consisting of a 300-500 nm thick layer of a transparent conducting oxide (TCO), a 80-200 nm thick layer of CdS deposited on top of the TCO layer, a 4-12 μm thick layer of CdTe on top of the CdS layer and a back contact layer formed by at least 100 nm thick layer of SB₂Te₃ and 100 nm thick layer of Mo. In particular, the transparent base substrate consists of soda-lime glass and the transparent conducting oxide is fluorine-doped (In₂O₃:F).

[0028] TCO layer consists of In₂O₃, which is doped with fluorine during the growth. The In₂O₃ target, differently from ITO, does not form any noodle. A very low resistivity is obtained by introducing in the sputtering chamber a small amount of fluorine in the form of a gaseous fluoroalkyle compound such as CHF₃ and a small amount of H₂ in the form of a mixture with an inert gas such as a Ar+H₂ mixture, in which H₂ is 20% in respect to Ar. A typical example is a 500 nm film of In₂O₃ deposited with a deposition rate higher than 10 Å/sec at a substrate temperature of 500° C., with an Ar flow-rate of 200 sccm, a CHF₃ flow-rate of 5 sccm and an Ar+H₂ flow-rate of 20 sccm. In this way, the reactive sputtering gas is composed by Ar containing 2.5 vol. % of CHF₃ and 1.8 vol. % of H₂. This film exhibits a sheet resistance of 5 Ω/_(square), a resistivity of 2.5 ×10⁻⁴ Ωcm and a transparency higher than 85% in the wavelength range of 400-800 nm. Another characteristic of this film is its good stability and the ability to stop Na diffusion from the soda-lime glass. This has been demonstrated by making CdTe/CdS solar cells on top of this type of TCO which have shown to be very stable even if heated up to 180° C. when illuminated by “ten suns” for several hours.

[0029] After deposition of the CdS film and CdTe film in the known way by sputtering or close-spaced sublimation, the CdTe film surface is treated with CdCl₂ in the following way.

[0030] 200 nm of CdCl₂ are deposited by evaporation on top of CdTe film with the substrate kept at room temperature. An annealing of 15-20 min is done at 400° C. in a vacuum chamber in which 500 mbar of Ar are introduced. After the annealing the chamber is evacuated keeping the substrate at 400° C. for 5 min. and, since CdCl₂ has a high vapour pressure at 400° C., any residual CdCl₂ re-evaporates from the CdTe surface.

[0031] CdO or CdTeO₃ are not formed since the annealing is conducted in an inert atmosphere, which does not contain O₂.

[0032] According to the present invention a Te-rich surface is not needed to obtain a non-rectifying contact if the contact is made by depositing on top of CdTe film a thin layer of a highly conducting p-type semiconductors such as Sb₂Te₃ or As₂Te₃. A good not rectifying contact is obtained on a clean CdTe surface if at least 100 nm thick layer of Sb₂Te₃ or As₂Te₃ is deposited by sputtering at a substrate temperature respectively of 250-300° C. and 200-250° C. Sb₂Te₃ grows naturally p-type with a resistivity of 10⁻⁴ Ωcm, while As₂Te₃ grows p-type with a resistivity of 10⁻³ Ωcm. The contact procedure is completed by covering the low resistivity p-type semiconductor with at least 100 nm of Mo, as common practice in the art. A thin layer of Mo is needed in order to have a low sheet-resistance on the back-contact.

[0033] As a source for the CdS and CdTe materials used to form the respective layers by sputtering or CSS a granulate material can be used, as common practice in this field. However, in view of the drawbacks mentioned above when operating in this way, according to a particularly preferred aspect of the invention a new sublimation source can be used which consists of a compact block obtained by melting and solidifying the material in an oven able to sustain a temperature higher than the melting temperature of the material. The procedure to prepare the CdS compact block is as follows: pieces of CdS are put in a graphite container of the desired volume together with boron oxide (B₂O₃), which is a low melting point material (450° C.) and exhibits a very low vapour pressure when melted. Since boron oxide has a density lower than that of CdS in the molten state, it floats over CdS and covers CdS completely upon cooling. In this way, CdS covered by B₂O₃, if it is put into an oven containing an inert gas at pressure higher than 50 atm, does not evaporate even at a temperature higher than its melting point. Since CdS melts at a temperature of 1750° C., the oven is heated up to a temperature of 1800° C. or more and then cooled down to room temperature. In this way, a unique compact block of CdS is obtained which results to be particularly suitable for use as a sublimation source in a close-spaced sublimation system. CdS films prepared with this type of source resulted to be very smooth and completely free of dust. The CdS films used to prepare the CdTe/CdS solar cells are typically 100 nm thick. The substrate temperature is kept at 200-300° C. when CdS is prepared by sputtering and at 480-520° C. when it is prepared by close-spaced-sublimation. The sputtered CdS layer needs an annealing at 500° C. in an atmosphere containing O₂ in order for the CdS/CdTe solar cell to exhibit a high efficiency. In the case CdS is prepared by close-spaced sublimation, O₂ is introduced in the sublimation chamber during the deposition. The role of O₂ is not known, but it is presumed to passivate the CdS grain boundaries.

[0034] In a preferred embodiment of the invention, the CdTe source too is a compact block obtained by melting and solidifying pieces of CdTe in an oven under high pressure as described previously. Since CdTe melts at 1120° C., the oven needs to be heated up to 1200° C. in order to have a complete melting of the CdTe pieces. CdTe films are deposited on top of CdS by close-spaced sublimation at a substrate temperature of 480-520° C. Deposition rates during the CdTe growth are typically 4 μm/min. In this way 8 μm of CdTe are deposited in 2 min.

[0035] By following the procedure described above several solar cells have been prepared by using as a substrate a 1 inch square low-cost soda-lime glass.

[0036] A typical area of these cells is 1 cm². The finished cells are generally put under 10-20 suns for several hours at a temperature of 180° C. in the open-circuit-voltage (V_(oc)) conditions. No degradation has been notified but rather a 20% or more increase in the efficiency has been found.

[0037] The efficiency of these cells are in the range 12%-14% with open-circuit-voltages (V_(oc)) larger than 800 mV, short-circuit-currents (J_(sc)) of 22-25 mA/cm² and fill-factors (ff) ranging from 0.6 to 0.66.

EXAMPLE

[0038] A cell exhibiting a 14% efficiency has been prepared in the following way: a soda-lime glass has been covered by 500 nm of In₂O₃:F (fluorine-doped) deposited at 500° C. substrate temperature as described above. 100 nm of CdS have been deposited by sputtering at 300° C. substrate temperature and annealed for 15 min at 500° C. in 500 mbar of Ar containing 20% of O₂. 8 μm of CdTe have been deposited on top of CdS by CSS at a substrate temperature of 500° C. Both CdS and CdTe films are produced from a compact block source as described above. A treatment with 150 nm of CdCl₂ has been done in an Ar atmosphere as described above. Finally a back contact has been created, without any etching, by depositing in sequence by sputtering 150 nm of Sb₂Te₃ and 150 nm of Mo.

[0039] After one hour under 10 suns at a temperature of 180° C. in open-circuit conditions the solar cell prepared in this way exhibited the following parameters: V_(oc) 852 mv J_(sc) 25 mA/cm² ff 0.66 efficiency 14%

[0040] The techniques used in this process such as sputtering and close-spaced sublimation are both fast, reproducible and easily scalable.

[0041] Sputtering systems capable to cover 1×0.5 m² area glass are already commercial, while close-spaced sublimation, which at a laboratory scale can cover easily 20×20 cm² area glass, does not give any problem in being scaled up to 1×0.5 m². An in-line process can work well if a large area glass is made to move slowly over the different sources namely TCO, CdS, CdTe, Sb₂Te₃ or As₂Te₃ and Mo. In order to make a series connection between the single cells the in-line system should also contain three laser scribing processes, the first after the TCO deposition, the second one before the back-contact deposition and the third one at the end of the process. An important part of the process according to the present invention is that no use is made of acids or liquids and, as a consequence, the process can operate on a continuous basis without the interruption which is needed to make the etching in acid or in a Br-methanol solution. 

1. A process for large-scale production of CdTe/CdS thin film solar cells, films of the cells being deposited, in sequence, on a transparent substrate, the sequence comprising the steps of: depositing a film of a transparent conductive oxide (TCO) on the substrate; depositing a film of CdS on the TCO film; depositing a film of CdTe on the CdS film; treating the CdTe film with CdCl₂; and depositing a back-contact film on the treated CdTe film; wherein the treatment of the CdTe film with CdCl₂ comprises the steps of forming a layer of CdCl₂ on the CdTe film by evaporation, while maintaining the substrate at room temperature; annealing the CdCl₂ layer in a vacuum chamber at a temperature generally within a range of 380° C. and 420° C. and a pressure generally within a range of 300 mbar and 1000 mbar in an inert gas atmosphere; and removing the inert gas from the chamber so as to produce a vacuum condition, while the substrate is kept at a temperature generally within a range of 350° C. and 420° C. whereby any residual CdCl₂ is evaporated from the CdTe film surface.
 2. The process set forth in claim 1, wherein the CdCl₂ layer is between about 100 nm and about 200 nm thick.
 3. The process set forth in claim 1, wherein annealing of the CdCl₂ layer is carried out for about 15-20 minutes.
 4. The process set forth in claim 1, wherein the inert gas is Ar.
 5. The process set forth in claim 1, wherein the back-contact film is formed of a Sb₂Te₃ layer covered by a layer of Mo.
 6. The process set forth in claim 5, wherein the Sb₂Te₃ layer is formed by sputtering at a temperature between about 250° C. and about 300° C.
 7. The process set forth in claim 1, wherein the back-contact film is formed of a As₂Te₃ layer covered with a layer of Mo.
 8. The process set forth in claim 7, wherein the As₂Te₃ layer is formed by sputtering at a temperature between about 200° C. and about 250° C.
 9. The process set forth in claim 1, wherein the transparent conductive oxide is In₂O₃ doped with fluorine.
 10. The process set forth in claim 9, wherein the TCO layer is formed by sputtering in an inert gas atmosphere comprising a mixture of hydrogen and a gaseous fluoroalkyle compound.
 11. The process set forth in claim 10, wherein a mixture of Ar and hydrogen is used which comprises between about 1% and about 3% hydrogen bv volume, and wherein the fluoroalkyle compound is CHF₃.
 12. The process set forth in claim 1, wherein, as a source material for formation of the CdS and the CdTe films by close-spaced sublimation, a CdS or, respectively, CdTe material generally in the form of a compact block is used.
 13. The process set forth in claim 12, wherein the compact block CdS (or CdTe) material is formed by covering pieces of CdS (or CdTe) material with boron oxide, heating the covered material to a temperature generally greater than its melting point in an inert gas atmosphere and at a pressure generally greater than 50 atm, and then cooling the material to room temperature, whereby the material is solidified in a generally compact block-like form.
 14. A process for large-scale production of CdTe/CdS thin film solar cells, films of the cells being deposited, in sequence, on a transparent substrate, the sequence comprising the steps of: depositing a film of a transparent conductive oxide (TCO) on the substrate; depositing a film of CdS on the TCO film; depositing a film of CdTe on the CdS film; treating the CdTe film with CdCl₂; and depositing a back-contact film on the treated CdTe film; wherein the transparent conductive oxide is In₂O₃ doped with fluorine.
 15. The process set forth in claim 14, wherein the TCO layer is formed by sputtering in an inert gas atmosphere comprising a mixture of hydrogen and a gaseous fluoroalkyle compound.
 16. The process set forth in claim 15, wherein a mixture of Ar and hydrogen is used which comprises between about 1% and about 3% hydrogen by volume and wherein the fluoroalkyle compound is CHF₃.
 17. A process for large-scale production of CdTe/CdS thin film solar cells, films of the cells being deposited, in sequence, on a transparent substrate, the sequence comprising the steps of: depositing a film of a transparent conductive oxide (TCO) on the substrate; depositing a film of CdS on the TCO film; depositing a film of CdTe on the CdS film; treating the CdTe film with CdCl₂; and depositing a back-contact film on the treated CdTe film; wherein as a source material for the formation of the CdS and the CdTe films by close-spaced sublimation, a CdS or, respectively, CdTe material generally in the form of a compact block is used.
 18. The process set forth in claim 17, wherein the compact block of CdS (or CdTe) material is formed by covering pieces of CdS (or CdTe) material with boron oxide, heating the covered material to a temperature generally greater than its melting point in an inert gas atmosphere and at a pressure generally greater than about 50 atm. and then cooling the material to room temperature, whereby the material is solidified in a generally compact block-like form.
 19. The process set forth in claim 1, wherein the transparent substrate is soda-lime glass.
 20. A CdTe/CdS thin film solar cell product of the process set forth in claim
 14. 